Recent attempts to detect intergalactic magnetic fields beyond
clusters, i.e. in even more rarefied regions of the intergalactic
space, have shown recent promise in imaging diffuse synchrotron
radiation of very low level. A very faint emission has been detected
at 327 MHz with the WSRT in the
Coma cluster region between the radio halo Coma C and
the relic 1253+275
[104].
The surface brightness of this diffuse emission is very low and it is only
enhanced at low frequency and low resolution, so it is only
visible as a positive noise in
Fig. 3. The existence of this
feature is confirmed by the asymmetric extension
of the central halo Coma C imaged at 1.4 GHz with the Effelsberg
single dish
[35],
and by the VLA data at 74 MHz
[36].
The equipartition magnetic field in this region
[105] is
10-7G. These
data may indicate the existence of a more widespread and somewhat lower
intergalactic magnetic field than in the ICM. It could be possibly
associated with large-scale shocks related to the formation of the
large-scale structure in the universe.

A possible evidence of magnetic field in the intergalactic medium
is found
[106]
in the filament of galaxies ZwCl 2341.1+0000 at z ~ 0.3.

The study of the RM of distant quasars could provide an independent
information on the magnetic field in the intergalactic medium. An
upper limit of
10-9 G for a cosmic magnetic field outside
clusters of galaxies has been derived in the literature
[1,
107].
However, this limit relies on several assumptions. New generation
instruments will shed light on this point.